The long-range goal of the research program proposed here is to contribute to our understanding of lipid metabolism in health and disease. The immediate objective will be to focus on the acyl-CoA carboxylases, including acetyl-CoA carboxylase, a key enzyme in the regulation of fatty acid biosynthesis, and propionyl-CoA, 3-methylcrotonyl-CoA, and geranyl-CoA carboxylases, enzymes involved in odd-chain and branched-chain fatty acid catabolism. Our aim is to undertake an intensive structure/function analysis of the catalytic and regulatory subsites of different acyl-CoA carboxylases from the same source in order to answer questions on a) the organization of subsites in these complexes; b) the degree of sharing of common structural features between pairs of homologous carboxylases (e.g., acetyl-CoA carboxylase versus propionyl-CoA carboxylase) from the same organism; c) the nature of the regulatory site(s) in acetyl-CoA carboxylase and its interaction with the catalytic sites; and d) the mechanism of assembly of multienzyme complexes and the advantages of the fusion of catalytic sites into multifunctional polypeptide chains. As a model for these general problems the acyl-CoA carboxylases of Pseudomonas citronellolis are under study to develop the techniques and affinity labeling reagents necessary to probe the molecular architecture of the highly integrated, multifunctional polypeptide chains present in the less tractable rat liver acetyl-CoA carboxylase and rat heart propionyl-CoA carboxylase.